1. Field of the Invention
The present invention relates to high frequency power amplifying circuit technology, and more particularly to a power amplifying circuit for a modulated high frequency signal having amplitude fluctuations, such as an OFDM (Orthogonal Frequency Division Multiplexing) signal used in wireless LAN (Local Area Network) or a QPSK (Quadrature Phase Shift Keying) signal used in CDMA (Code Division Multiple Access) mobile telephony.
2. Description of the Related Art
Recently, signal processing techniques used in radio communications are a digital modulation/demodulation scheme; and schemes using modulation involving signal amplitude fluctuations have become a mainstream technique from a viewpoint of improving frequency use efficiency. For example, QPSK (Quadrature Phase Shift Keying) is used in CDMA (Code Division Multiple Access) mobile telephony; OFDM (Orthogonal Frequency Division Multiplexing) digital modulation scheme is used in IEEE (Institute of Electrical and Electronic Engineers) communication standard “802.11a/g”—compliant wireless LAN (Local Area Network). Signals handled in these modulation schemes are each a signal involving amplitude fluctuations.
In order to output as radio wave with no “distortion” such a signal involving amplitude fluctuations, there is needed a linear high frequency power amplifying circuit which has a constant gain irrespective of the amplitude of input signal. For example, Japanese Patent Laid-Open No. 7-193435 (Patent Document 1) has proposed a high frequency amplifying circuit which corrects the bias applied to the transistor in accordance with “distortion components” contained in an input signal, whereby satisfactory linearity can be maintained over a broad band.
In general, as the linear high frequency power amplifying circuit, there has been broadly used a “class A amplifying circuit” which amplifies the whole input signal. However, in the class A amplifying circuit, even when a small signal is inputted, power consumption does not decrease, so its power efficiency is significantly low when used as an amplifying circuit for a signal having amplitude fluctuations. Particularly, this presents a large practical problem that battery drain is noticeable when a class A amplifying circuit is used as the high frequency power amplifying circuit of a mobile radio communications device using a battery as its power source.
To avoid this drawback of class A amplifying circuit, a class AB or class B amplifying circuit can be used. However, in both the class AB amplifying circuit and class B amplifying circuit, the operating point of transistor moves as input electric power varies, so the gain exhibits non-linearity (deviation from a proportional relation between input and output) such as a variation depending on input electric power. To avoid this problem, the “distortion factor correction” technique disclosed in Patent Document 1 can be used; but in this case, the scale of a control circuit for performing such correction is large, thus increasing manufacturing cost, which is an additional problem.